Spotify is a leader in the music streaming market, but the space is crowded with several others, including Pandora, Apple's iTunes Radio, Google Play Music All Access and Rdio. And more services are on their way. Beats by Dre this week announced an upcoming music streaming service, and YouTube is rumored to have a similar one coming next year.

For Spotify, a free mobile version could help it stand out. The ad-supported mobile service could attract users who could one day be converted into paying customers, Bloomberg reports.

According to the report, which cites an unnamed source, the way the free mobile service will work is by letting users choose 15 tracks to play on their smartphones. The tracks will play in random order, and Spotify will throw in ads between songs. If users don't pick 15 songs, Spotify will add random songs to fill the spots.

But each song can be played a limited number of times, pushing users to continually choose new songs. Users will be required to stream the songs from the Internet -- they won't be able to save the tracks on their devices, which is a feature that's available to paying customers.